(a) Field of the Invention
The present invention relates to a method for manufacturing a conductive element substrate. In particular, it relates to a method for manufacturing reflecting electrodes on an active matrix substrate for forming a liquid crystal display.
(b) Description of Related Art
Semi-transmissive liquid crystal displays, which are one of liquid crystal displays, are capable of displaying in both transmissive and reflective modes. The semi-transmissive liquid crystal displays are given with a feature of a transmissive liquid crystal display such as high visibility in dark place owing to a backlight installed therein and a feature of a reflective liquid crystal display such as power saving by using ambient light.
A common liquid crystal display includes an active matrix substrate on which a plurality of pixel electrodes and TFTs (thin film transistors) are arranged in matrix configuration, a counter substrate provided with common electrodes and a liquid crystal layer sandwiched between the substrates. By writing certain charge to each of the pixel electrodes, a certain voltage is applied to a liquid crystal capacitor constituted of the liquid crystal layer between the pixel electrode and the common electrode. Visual display is achieved by making use of changes in orientation of liquid crystal molecules in response to the applied voltage.
In the semi-transmissive liquid crystal display, each of the pixel electrodes, which constitutes a minimum unit called pixel for forming a visual image, is formed of a transparent electrode and a reflecting electrode. The transparent electrode allows light from the backlight to pass through to execute display in the transmissive mode, while the reflecting electrode reflects ambient light to execute display in the reflective mode.
The transparent electrode is made of a transparent conductive film such as a tin oxide film, a zinc oxide film, an ITO (Indium Tin Oxide) film made of a compound of indium oxide and tin oxide and an IZO (Indium Zinc Oxide) film made of a compound of indium oxide and zinc oxide. In particular, the ITO film and the IZO film have been commonly used because of their excellent visible light transmission and favorable conductivity.
As the reflecting electrode, a conductive metal film such as an aluminum film has been commonly used because of its high reflectance and low electrical resistance.
In general, electrode materials such as the transparent conductive film and the conductive metal film described above have different work functions. Therefore, the transparent electrode and the reflecting electrode made of different materials will have different work functions. In this case, the transparent and reflecting electrodes show different electrode potentials, which possibly leads to defective display. The reason therefor is described below.
Considering the lifetime of liquid crystal molecules, liquid crystal displays are AC-driven to alternate the polarity of voltage applied to the liquid crystal layer. However, if the voltage applied to the liquid crystal layer is distorted by parasitic capacitance of TFTs or the like, a direct voltage may possibly be applied to the liquid crystal layer. Therefore, it is also necessary to apply an offset voltage to the liquid crystal layer.
The offset voltage is given only to the whole pixels and cannot be applied one by one. Therefore, if the reflecting electrode and the transparent electrode are different in electrode potential as described above, the offset voltage is given only to one of the electrodes. In such a case, a direct voltage is applied to the liquid crystal layer to cause variations in light intensity (flicker), resulting in significant decrease in display quality.
To solve the defective display (display misalignment) derived from the difference in work function, attempts have been made to equalize the work functions of the materials for the reflective electrode and the transparent electrode.
For example, paying attention to the work functions of electrode materials, Japanese Unexamined Patent Publication HEI10-206845 describes a technique of reducing flicker of a reflective liquid crystal display by providing a common electrode and a reflecting electrode (pixel electrode) sandwiching a liquid crystal layer with an almost equal work function.
Further, for a semi-transmissive liquid crystal display including a transparent electrode made of an ITO film and a reflecting electrode made of an aluminum film, there has been a known technique of forming a transparent conductive film having a work function close to that of the ITO film on the aluminum film, thereby equalizing the work functions of an electrode material at the reflective electrode surface and an electrode material at the transparent electrode surface.
To prevent the occurrence of display misalignment between the reflective mode and the transmissive mode, an IZO film having a work function close to that of the ITO film is formed on the aluminum film forming the reflecting electrode in a pixel. The reason why the ITO film is not formed on the aluminum film is that the aluminum film and the ITO film bring about electrolytic corrosion when they come into to contact in the course of electrode formation and the ITO film is dropped off.
The provision of the IZO film as an uppermost layer of the reflecting electrode is advantageous in that the IZO film is a transparent conductive film and does not hinder the function of the aluminum film as the reflecting electrode, the work function of the IZO film is close to that of the opposed transparent electrode and the IZO film can be patterned with an etchant used for etching the underlying aluminum film.
On the substrate provided with the reflecting electrodes (pixel electrodes), other various metal wires are formed. For example, a driver (drive circuit) is formed at the end of the substrate to input a drive signal from outside. A contact terminal electrode is also formed at the end of the substrate to connect the driver and a wire for feeding a voltage to the pixel electrodes. As an uppermost layer of the contact terminal electrode, an ITO film having stability to air and low contact resistance is used. If wires and electrodes made of an aluminum film are formed on the substrate provided with the ITO film of the contact terminal electrode, electrolytic corrosion occurs upon contact between the aluminum film and the ITO film as described above. As a result, the ITO film comes off.
To solve the problem of electrolytic corrosion caused by contact between the aluminum film and the ITO film, a molybdenum film is formed as a protective metal film between the aluminum film and the ITO film.
It has been known that there is no need to use different etchants to etch the aluminum film and the molybdenum film because they can be patterned with the same etchant (e.g., a mixed solution of nitric acid, phosphoric acid, acetic acid and water). For example, Japanese Unexamined Patent Publication No. 2000-148042 discloses a method for patterning a two-layered film made of an aluminum film and a molybdenum film by spraying a single etchant thereon in the film thickness direction such that the patterned film is substantially tapered upward when viewed in cross section.
Further, it has also been known that the IZO film, aluminum film and molybdenum film can be patterned with the same etchant (e.g., a mixed solution of nitric acid, phosphoric acid, acetic acid and water) and there is no need of using different etchants for each of the layers.
As described above, from the viewpoints of visible light transmission, conductivity and compatibility with the underlying aluminum film (in respect of etching and electrolytic corrosion), an amorphous IZO film is suitably used as the transparent conductive film formed on the aluminum film serving as the reflecting electrode.
However, if a conductive metal film (aluminum film) and an amorphous transparent conductive film (IZO film) are formed in sequence and then the laminated conductive film including these two films are etched using a patterned resist as a mask, the edge portions of an upper amorphous transparent conductive layer 6b″ may possibly remain protruding more outward than the edge portions of a conductive metal layer 6a as shown in a schematic sectional view of FIG. 45.
This is presumably because of the nature of the IZO film which is less likely to be etched by a weakly acid etchant used for etching the aluminum film than the aluminum film.
Specifically, when an IZO layer is formed as an uppermost layer to prevent display misalignment, an aluminum layer is formed as a middle layer serving as the reflecting electrode and a molybdenum layer is formed as an undermost layer to prevent electrolytic corrosion, the middle aluminum layer and the undermost molybdenum layer are etched faster than the uppermost IZO layer. Therefore, if these layers are patterned at one time using the same etchant, the resulting layered structure shows a cross section which is substantially tapered downward from the farthest layer from the substrate as shown in a sectional view of FIG. 46. Thus, the IZO layer is reduced in strength and likely to come off.
Referring to FIG. 46, the molybdenum layer 102, the aluminum layer 103 and the IZO layer 104 are formed on a glass substrate 101 in this order. The middle aluminum layer 103 is so patterned that its cross section becomes narrower than that of the uppermost IZO layer 104. Detailed explanation about this phenomenon is given with reference to FIGS. 47 to 52.
Referring to FIG. 47, an uppermost IZO film 104, a middle aluminum film 103 and an undermost molybdenum film 103 are formed on the glass substrate 101 to form a three-layered structure. On the uppermost IZO film 104, a resist layer 105 patterned in a desired configuration is formed. If these films are etched in this state, the IZO film 104, aluminum film 103 and molybdenum film 102 are etched almost isotropically.
As shown in FIG. 48, the uppermost IZO film 104 is first etched in the thickness direction. Then, after the etching of the IZO film 104 is finished, the aluminum film 103 is etched in the thickness direction and at the same time, the IZO film 104 is etched in the horizontal direction as shown in FIG. 49.
After the etching of the aluminum film 103 is completed, the molybdenum film 102 is etched in the thickness direction and at the same time, the aluminum film 103 is etched in the horizontal direction. At this time, since the aluminum film 103 and the molybdenum film 102 are much higher in etch rate in both of the thickness and horizontal directions than the IZO film 104 in the horizontal direction, the etching of the aluminum film 103 and the molybdenum film 102 proceeds much faster in both of the thickness and horizontal directions than the etching of the IZO film 104 in the horizontal direction. FIGS. 48 to 50 indicate the degree of etch rate by the number of arrows.
When the etching of the molybdenum film 102 in the thickness direction is completed, the resulting aluminum layer 103 and molybdenum layer 102 are more narrowed in the horizontal direction than the uppermost IZO layer 104 as shown in FIG. 51.
FIG. 52 is a sectional view illustrating the layered structure after the removal of the resist film 105. By any etching method (showering, dipping, combination of showering/dipping), the underlying aluminum layer 103 and molybdenum layer 102 which have been etched at a higher etch rate are more narrowed when viewed in cross section than the IZO layer 104 which has been etched at a lower etch rate, as shown in FIG. 52. Thus, the resulting layered structure is substantially tapered downward from the farthest layer from the substrate.
If the edge portions of an amorphous transparent conductive layer 6b″ (the IZO layer 104) for forming the reflecting electrode remain protruding more outward than the edge portions of the underlying layer, the edge portions of the amorphous transparent conductive layer 6b″ may possibly come off in later steps which applies a load on the substrate surface, such as rubbing, and flakes of the amorphous transparent conductive layer 6b″ may possibly adhere to the pixel electrodes on the substrate. In this case, a short circuit occurs between the pixel electrodes to decrease a manufacturing yield of the active matrix substrate.
That is, in the formation of conductive elements by patterning a laminated conductive film including a first conductive metal film made of an aluminum film and a molybdenum film and a second conductive metal film made of an IZO film which is lower in etch rate than the first conductive metal film, the edge portions of the second conductive metal film (IZO layer) may possibly come off.